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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

SILVER ALLOYS

28*

precipitate all the silver has been added to a solution of silver in nitric acid,
a cloud will be formed in the liquid by addition of either sodium chloride or
silver nitrate. This may constitute a source of error if, as Gay-Lussac originally
suggested, a weak standard silver nitrate solution is used in cases where the
amount of sodium chloride added is in excess with respect to the silver dissolved.
On this account Stas i proposed a modification of the Gay-Lussac method, this
consisting in the replacement of the sodium chloride solutions by the correspond-
ing hydrobromic acid solutions, silver bromide being perfectly insoluble. This
method, which is used in the Brussels mint, is rather more delicate than that
of Gay-Lussac, but it requires perfect expulsion of the nitrous vapours and
special precautions to prevent access of light to the tests, etc.

If, however, the Gay-Lussac method is carried out as described above, the
weak standard silver nitrate being suppressed and identical conditions employed
in both control test and that on the sample, all causes of error are eliminated.

As regards the influence of extraneous metals, it must be borne in mind that
mercury affects the accuracy of the results, as it also is precipitated as mercurous
chloride ; when this metal is present, it is expelled by heating the alloy to fusion
in a graphite crucible. When tin, antimony or bismuth is present, an opalescent
liquid is obtained in which it is difficult to observe the formation of cloudiness.
In presence of antimony or bismuth a little tartaric acid (1-2 grams) is added,
whilst when the alloy contains tin or a large proportion of lead, it is advisable
to dissolve it in sulphuric acid. Further, gold in proportion exceeding a fineness
of 60-80 (6-8%) influences the results as it withholds a little silver. Copper in
amount greater than 50% gives coloured solutions, which render observation
difficult.

2.  Determination of the Gold.—In a flask with a long, narrow neck,
10 grams of the sample are dissolved in 80-100 c.c. of nitric acid (D 1-2),
the solution being decanted off and the residue again boiled with nitric
acid, which is also decanted.    The residue is washed repeatedly with hot
water by decantation and the undissolved gold remaining as a black powder
collected in a refractory, unglazed crucible (see Gold and its Alloys—Quarta-
tion), dried, ignited and weighed.

3.  Detection of Tin, Antimony, Copper, Bismuth and Lead.—
5 grams of the sample are treated with nitric acid (D 1-2).    In presence
of tin or antimony, the liquid is opalescent or contains a slight white pre-
cipitate.    If the filtered solution is rendered alkaline with ammonia, it
will turn more or less intensely blue if copper is present, whilst a flocculent
precipitate will form in presence of bismuth or lead, which may be identified
by the usual means.

*

* *

Natural and crude silver often contains small quantities of gold, lead, mercury,
copper, antimony, arsenic and sometimes selenium and bismuth. The presence
of bismuth is extremely harmful, alloys made with silver containing only traces
of bismuth being rough and brittle. Refined silver is usually very pure, the
best qualities containing, on the average, 99-9% Ag.

Silver is used especially alloyed with copper for jewellery, coinage, etc. The
legal standard for Italian 5-lire pieces is 900 fine with a variation of 2 either way,
and that for plate is usually 900 fine, and that for jewellery 800 or even less—
down to 500 fine.

1 De Koninck :   Trait6 de chimie analyiique, II, p. 561. crown of the muffle.l and ether, dried at